1. Field of the Invention
The present invention generally relates to a circuit for driving an organic light emitting diode (hereinafter referred to as “OLED”) display, and in particular certain embodiments of the present invention relate to a dual-scan circuit for driving an OLED display device.
2. Description of the Related Art
An OLED display device is an electro-luminescence device. Generally, an OLED display device is made of a stack of layers on a glass substrate. The stack of layers is generally formed by an anode electrode, a hole-injection layer, a light-emitting layer, an electron-injection layer, and a cathode electrode in sequence. The anode electrode is an electrically conductive material formed and patterned on the glass substrate.
When a current source is applied to the OLED display device, there is an electric potential difference between the anode electrode and the cathode electrode. The holes in the hole-injection layer will be pushed toward the direction of the cathode electrode, and the electrons in the electron-injection layer will be pushed toward the direction of the anode electrode. The holes and the electrons combine in the light-emitting layer, and light with a specific wavelength is emitted from the light-emitting layer. It is worth noting that the light intensity emitted by the OLED display device is proportional to the driving current of the current source.
An OLED display device includes an array of pixels. For example, an OLED display device with an array of n*m pixels contains n rows and m columns of pixels. Each pixel is connected to a specific row line and a specific column line. Hence, the OLED display device with an array of n*m pixels contains “n” numbers of row lines and “m” numbers of column lines. The row lines and the column lines are connected to the current source(s), e.g., one or more driving chips. The intersection of each row line and each column line locates a pixel, which contains an OLED.
The “dual-scan” method is a technique for shortening an addressing period. According to the technique, the array of pixels is divided into two groups, using two independent driving chips to drive the two groups separately. Referring now to FIG. 1, a schematic circuit diagram of a traditional OLED display device using the dual-scan method is shown. The traditional OLED display device contains an array of n*m pixels, each of which is connected to a specific row line and a specific column line. Each pixel contains an OLED 10.
Referring to FIG. 1, the first column of pixels is connected to the first column line, shown as “CL1”, and the second column of pixels is connected to the second column line, shown as “CL2”. The final, m-th column of pixels is connected to the m-th column line, shown as “CLm”. Each of the column lines in the upper half of the array is connected to the first driving chip 20, and each of the column lines in the lower half of the array is connected to the second driving chip 30.
Referring again to FIG. 1, the first row of pixels is connected to the first row line, shown as “RL1”, and the second row of pixels is connected to the second row line, shown as “RL2”. The (n/2)-th column of pixels is connected to the (n/2)-th row line, shown as “RLn/2”. All of the RL1 to RLn/2 row lines are connected to the first driving chip 20. In other word, the upper half of the pixels of the OLED display device is driven by the first driving chip 20.
Still referring to FIG. 1, the (n/2+1)-th row of pixels is connected to the (n/2+1)-th row line, shown as “RL(n/2+1)”, and the n-th row of pixels is connected to the n-th row line, shown as “RLn”. All of the RL(n/2+1) to RLn row lines are connected to the second driving chip 30. In other word, the lower hall of the pixels of the OLED display device is driven by the second driving chip 30.
In accordance with the dual-scan method two rows can be selected simultaneously. One advantage of this is that the addressing period is half of that in the single scan method in which only one row is selected at one time. However, there are generally unavoidable electric characteristic differences between the two driving chips 20, 30 because of unavoidable manufacture deviations, different operation temperature, and/or different power levels of the current sources. The electric characteristic differences may result in different driving currents being provided by the different driving chips. As mentioned above, such different driving currents would cause different light intensities to be emitted from the OLED pixels. This, in turn, would result in different brightnesses between the upper part of the OLED display device and the lower part of the OLED display device.
In other words, by using the traditional dual-scan method, the upper part of the OLED display device is frequently brighter than the lower part of the OLED display device, or vice versa. This brightness difference reduces the value of the OLED products and also is disliked by users.
FIG. 2 shows a schematic diagram of the brightness difference of an OLED display device driven by the traditional dual-scan method. The “white circles” in FIG. 2 represent “brighter pixels”, and the “black circles” in FIG. 2 represent “darker pixels”. FIG. 2 shows that the upper half of the OLED display device is brighter than the lower half of the OLED display device. The reason is that the driving current of the first driving chip, which drives the upper half, is larger than that of the second driving chip, which drives the lower half of the OLED display device. This is due to the unavoidable electric characteristic differences between the two driving chips mentioned above. Such a brightness difference would be disliked by users.
Therefore, there is a need to create a new dual-scan circuit and method to overcome the above-mentioned problem.